Lung volumes in COPD: not only the total lung capacity.

We read with extreme interest the recently published work by O’Donnell and colleagues (May 2010). 1 Comparing lung volumes obtained using different techniques in a sample of patients with severe airfl ow limitation, the authors conclude that plethysmography systematically overestimates lung volumes with regard to gas dilution and thoracic imaging techniques. We believe it is meritorious to assess how the available diagnostic methods differ in determining lung volumes in the context of severe bronchial obstruction. However, we think the authors might have derived more from their data and results. We are afraid the implementation of the present study results in the clinical setting might be limited. In fact, the main reason for measuring lung volumes in subjects with COPD is to determine the presence and degree of lung hyperinfl ation. To the radiologist, hyperinfl ation of the lungs implies an increase in total lung capacity (TLC) because this is the lung volume at which chest radiographs are normally obtained. In a clinical context, however, hyperinfl ation implies an abnormal increase in the volume of gas in the lungs at the end of tidal (functional residual capacity [FRC]) or maximal (residual volume [RV]) expiration. 2 Moreover, hyperinfl ation is sometime inferred from an increase in the RV TLC (Motley index) and the FRC TLC ratios, commonly used as a surrogate of air trapping. Unfortunately, the authors do not mention any of these other parameters in their study. This is, in our opinion, a major concern because TLC in COPD patients varies as a function of the prevalent phenotype (increased in emphysema, often normal in chronic bronchitis); therefore, it cannot be used routinely to address pulmonary hyperinfl ation in COPD. Conversely, RV and FRC are strongly related to the severity of airfl ow obstruction. On the one hand, Dykstra and colleagues 3 did not fi nd a significant association between TLC and the degree of airway obstruction. On the other hand, such a relationship is well established between RV TLC or FRC TLC, proper indexes of hyperinfl ation, and the severity of bronchial obstruction. 4 , 5 Moreover, to further complicate the clinical interpretation of lung volume changes in COPD, it can be observed that RV increase usually antedates that of FRC and then TLC. 6